Add new opaque type predicate

[oli-obk]

It's not emitted yet and most logic for it doesn't exist.
This commit exists solely to make future changes smaller

r? @jackh726

this is one of the changes I mean in #92007 (comment)

Merging this will make further work easier to review and less bitrotty, but this PR has no logic changes to the actual execution of the compiler.

[rust-log-analyzer]

The job mingw-check failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

    Checking tracing-tree v0.1.9
    Checking rustdoc-json-types v0.1.0 (/checkout/src/rustdoc-json-types)
    Checking tera v1.10.0
    Checking rustdoc v0.0.0 (/checkout/src/librustdoc)
error[E0004]: non-exhaustive patterns: `OpaqueType(_, _, _)` not covered
   --> src/librustdoc/clean/mod.rs:278:15
    |
278 |         match bound_predicate.skip_binder() {
    |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ pattern `OpaqueType(_, _, _)` not covered
   ::: /checkout/compiler/rustc_middle/src/ty/mod.rs:615:5
    |
    |
615 |     OpaqueType(Ty<'tcx>, Ty<'tcx>, bool),
    |
    = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
    = note: the matched value is of type `PredicateKind`

[jackh726]

Is there any documentation as to why this is needed? If there is, can you made add that here somewhere? If not, can you explain a bit?

[jackh726]

Why not just make the left be the opaque type being defined, and get rid of the bool?

[oli-obk]

I think I tried that but got messed up diagnostics, but I'll try again

[oli-obk]

Closing in favour of merging #92007 in one go by allocating a pair-review session with niko and lcnr